Learn French: En général, je travaille le matin. - In general, I work in the morning.

Yes! Even though it is two words, it functions just like a single-word frequency adverb (like souvent or parfois) to tell us how often something happens. It acts just like the English words generally or usually.

I learned that adverbs usually go right after the verb. Why is en général at the beginning here?

Short adverbs like souvent (often) usually go right after the verb, as in Je travaille souvent. However, longer frequency phrases like en général are flexible and are very commonly placed at the very beginning of a sentence to set the scene.

Why is it just le matin? Do we need a word for in to say in the morning?

In French, you do not need a preposition for general parts of the day. You simply use the word le plus the time of day. So, le matin translates to in the morning and le soir translates to in the evening.

Does le matin here mean this morning or every morning?

It means in the morning as a regular habit (every morning). Using le before a part of the day expresses a recurring routine, which matches perfectly with the phrase en général. If you wanted to say this morning as a one-time event, you would use ce matin.