Мы обсудили миссию.

Because it is the direct object of the verb обсудили (discussed), so it takes the accusative case.

• Dictionary form: миссия
• Accusative singular: миссию

This happens because обсудить takes a direct object without a preposition.

So:

• Мы обсудили миссию = We discussed the mission

Since миссия is a feminine noun ending in -я, its accusative singular changes to -ю.

Обсудили is the past tense plural form of обсудить, which means to discuss, to talk over, or to go over something and reach some kind of conclusion.

In this sentence, обсудили means:

• we discussed
• we talked over

The ending -ли shows past tense plural, matching мы (we).

This is a very common aspect question.

• обсудить = perfective
• обсуждать = imperfective

Мы обсудили миссию uses the perfective verb, which suggests the discussion is viewed as a completed whole.

So it means something like:

• We discussed the mission
• We finished discussing the mission

If you said Мы обсуждали миссию, that would focus more on the process:

• We were discussing the mission
• We discussed the mission (with emphasis on the activity, not necessarily the result)

Russian has no articles, so there is no direct equivalent of the or a/an.

So миссию can mean:

• the mission
• a mission

The exact meaning depends on context. In your sentence, the translation is probably the mission because the mission is already known from the situation.

Yes, it can often be omitted.

Russian past tense verbs already show number and sometimes gender, so обсудили already tells you the subject is plural.

That means both of these are possible:

• Мы обсудили миссию.
• Обсудили миссию.

Including мы makes the subject explicit and can add emphasis or clarity:

• We discussed the mission.

Without мы, it can sound a little more natural in context if the subject is already obvious.

Yes. Russian word order is more flexible than English word order.

The neutral order here is:

• Мы обсудили миссию.

But other orders are possible, depending on emphasis:

• Миссию мы обсудили. = As for the mission, we discussed it.
• Обсудили мы миссию. = marked or contrastive, depending on context

So the basic meaning stays similar, but the focus changes.

The stress is:

• обсуди́ли
• ми́ссию

So the full sentence is pronounced approximately:

• my ahbsu-DEE-lee MEE-see-yu

A few helpful notes:

• Мы sounds like my but shorter and with a Russian ы sound.
• In обсуди́ли, the stress is on -ди́-.
• In ми́ссию, the stress is on the first syllable.

It is:

• past tense
• plural
• from the verb обсудить
• perfective aspect

Russian past tense is formed differently from English. In the plural, past tense verbs usually end in -ли:

• я/он/она обсудил / обсудила
• мы/вы/они обсудили

Because the subject is мы, the verb must be обсудили.

Yes, миссия is a feminine noun.

You can tell from its dictionary form ending -я, which is typical for many feminine nouns.

That matters because feminine nouns change in certain ways by case. Here:

• nominative: миссия
• accusative: миссию

So the noun’s gender helps determine how it declines.

Usually, yes. Обсудить normally takes a direct object in the accusative case.

Examples:

• обсудить план = to discuss the plan
• обсудить проблему = to discuss the problem
• обсудить миссию = to discuss the mission

So in your sentence, миссию is exactly the form you would expect after обсудили.

Yes. Миссия can be used in several senses, for example:

• a military or space mission
• a diplomatic mission
• a religious mission
• an organization’s or person’s mission/purpose

So the exact shade of meaning depends on context. Grammatically, though, the sentence works the same way in all of these meanings.