Возле лавочки стоит урна, поэтому после пикника легко убрать мусор.

Because возле requires the genitive case.
The base form is лавочка (bench), but after возле (near/by) it becomes лавочки.

So:

• лавочка = a bench
• возле лавочки = near the bench

This is a very common pattern in Russian: certain prepositions always require a particular case.

Возле means near, beside, or by. In this sentence, it means the trash bin is located near the bench.

It is very similar to около, and in many situations they are interchangeable:

• возле лавочки
• около лавочки

Both mean near the bench.
The difference is usually very small and often just stylistic or based on speaker preference.

Стоит is from стоять, meaning to stand.

Russian often uses verbs of position for objects:

• стоит = stands
• лежит = lies
• висит = hangs

So Возле лавочки стоит урна literally means A bin stands near the bench.

In natural English, we usually just say There is a bin near the bench, but Russian often prefers this more physical way of describing location.

In the present tense, Russian usually does not use a direct equivalent of English there is / there are.

So instead of saying something like There is a bin near the bench, Russian commonly says:

• Возле лавочки стоит урна
  literally: Near the bench stands a bin

This is completely normal Russian. The idea of existence is understood from the context and word order.

Here урна means a trash bin / litter bin / waste bin, especially an outdoor public bin.

Be careful: урна can also mean urn in other contexts, such as a funeral urn or a ballot box.
But in this sentence, because of picnic and trash, it clearly means a bin for rubbish.

Because после (after) also requires the genitive case.

The dictionary form is пикник, but after после it becomes пикника:

• пикник = picnic
• после пикника = after the picnic

So both возле and после are good examples of prepositions that govern the genitive.

Russian often uses легко + infinitive to mean it is easy to do something.

So:

• легко убрать мусор = it is easy to clean up the trash

There is no explicit subject like it in Russian here. This is a common impersonal structure.

You could also say:

• можно легко убрать мусор = you can easily clean up the trash

But that shifts the meaning slightly toward it is possible rather than simply it is easy.

Убрать is the perfective form, while убирать is imperfective.

Here, the sentence refers to a completed action: to clean up / remove the trash after the picnic. That is why perfective убрать is natural.

Compare:

• убирать мусор = to be cleaning up trash / to clean up trash in general
• убрать мусор = to clean up the trash completely, as one finished action

Because the sentence talks about the result after the picnic, убрать fits better.

Мусор is the direct object of убрать, so it is in the accusative case.

But мусор is an inanimate masculine noun, and for many such nouns the accusative singular looks the same as the nominative singular:

• nominative: мусор
• accusative: мусор

So the case has changed grammatically, but the form stays the same.

Russian word order is flexible, and the beginning of the sentence often sets the scene.

So:

• Возле лавочки стоит урна

starts with the location: Near the bench...
Then it introduces what is there: ...there is a bin / a bin stands.

This sounds natural because it first tells you where, then what.

A different order is possible, for example:

• Урна стоит возле лавочки

This is also correct, but it puts a little more focus on урна itself.

Because поэтому means therefore / so / that’s why, and it connects the second clause to the first.

The sentence has two parts:

• Возле лавочки стоит урна
• поэтому после пикника легко убрать мусор

Russian normally separates such clauses with a comma, especially when the second part expresses a result or conclusion.

So the comma helps show the logic:

• there is a bin near the bench,
• therefore it is easy to clean up the trash after the picnic.