Learn Dutch: Meestal wandelen onze vrienden.

This happens because the sentence starts with the time word meestal (usually). In Dutch, the main verb must stay in the second position. When something other than the subject takes the first spot, the subject has to move right after the verb to make room. This is the inversion you are currently learning.

What is the normal word order if I start the sentence with the subject?

If you put the subject first, there is no inversion. The sentence becomes Onze vrienden wandelen meestal. Putting meestal at the front like in this lesson's sentence just gives the time word a bit more emphasis.

Why do we use onze for "our" instead of ons?

The word for "our" depends on the noun that follows it. We use ons only for singular het-words. Because vrienden (friends) is plural, it acts like a de-word, meaning we must use onze.

Why does the verb wandelen end in -en here?

The verb must match the subject. The subject is onze vrienden (our friends), which is plural. For plural subjects, Dutch verbs use their full plural form, which almost always ends in -en.