What is assimilation?

Assimilation is where, in spoken language, one sound is replaced for another to make it similar to other sounds in the word/phrase. There are a few different types of assimilation but to explain the concept let's stick to assimilation of place. Try saying p, b, m. Did you notice how they are all made by pressing your lips together (they are bilabials) and how your mouth didn't need to move a lot to change between them? Now say n. Notice how now your lips are apart and your tongue is touching the roof of your mouth. Because these sounds are articulated (made) in different places in the mouth it is more physically demanding to move between them. This may not seem too difficult in isolation, but when you're saying strings of sounds together in words and phrases you're asking your mouth to do quite a lot and can't blame it for cutting corners. This is why we replace sounds with those articulated in the same place as others in the word or phrase.
For example, say 'green paint' as normally as you can (maybe repeat it a few times) and notice that your tongue doesn't touch the roof of your mouth at the end of 'green'. Instead you bring your lips together and make a m sound in preparation for the p, as both of them are made with the lips together. It will be even easier to notice this if you intend to say it all then stop yourself before 'paint'. The same process happens when children learn to speak as it is much easier to produce similar (or the same) sounds than to produce lots of different ones and it takes time to build the skill to change easily between mouth positions. For example, 'rabbit' is often said as 'babbit' so that the first consonant is now bilabial like b, meaning it is much less effort to say the whole word. This is also evident in language change as words like 'impossible' were originally like 'inpossible' but were altered in the same way. Therefore, this process is useful in multiple areas of your course from phonetic transcriptions, through child language and into language change.