I am a convert from the Evangelical Church and my RCIA teacher told me that the seven days of creation in Genesis is a primitive mode of speaking and does not really mean seven days. Is this true?

The catechism discourages the word “primitive” (e.g. CCC #304) in reference to scripture. A better Catholic approach is to describe the creation account as a poetic account, rather than a modern, scientific account.
 
If we try to hold to a literalistic reading of all the details, we have difficulties. For, if a “day” is a 24 hour cycle of the sun, it is surprising to read that the sun and moon are not even created until the 4th “day”.
 
Further, if the account is purely a literalistic, scientific account, there is the problem that there are two accounts of creation in Genesis 1-2:3 & 2:4ff. They are both very different.
 
What is essential for us to hold from these accounts is that God made everything out of nothing, He did so in wisdom and love, guiding every step. And though transcendent, remains present and active in all he has made.
 
So, seven days is likely more poetic than scientific.