Are codons that map to the same amino acids interchangeable?
I think given that you're just getting started with genetics, you can say that the codons are interchangeable. This is generally true, though not technically correct. Here are a few reasons for why this is the case, though there's probably more:


Specific organisms use specific codons with different frequencies. This is usually related to the tRNA abundance in the cell. Using a codon for which there is relatively less tRNA available would be expected to lead to a decreased protein synthesis rate. 
Using different codons can affect mRNA secondary structure, which could affect expression. 
Some codons, especially stop codons, can influence translation contextually through a process called recoding. This can include frame-shifting, read-throughs, cleavage or, perhaps most famously, coding for the amino acid selenocysteine and much more. See here.
Different codons can also influence translation contextually through pausing. This affects the rate of protein synthesis as well as its folding. See here.
Changing codons can affect regulatory sequences such as miRNA binding sites. Though these are usually present in the UTRs, they can also occur in the CDS.