To truly understand the difference between FLAC and MP3, you need to understand the difference between lossy and lossless files. Take an example of the images you see on the internet. These images come in a compressed format. In order for them to be displayed to you, some parts of the images have to be thrown away, to allow it to be compressed. On the flip side, the lost part can never be gotten back- it is lost forever.
This is the case of the lossy nature of MP3s. It is typically compressed, with part of it shaved off. Once you try converting MP3 to WAV, you will barely get the shaved off part. On the flip side, if you try converting the WAV file back to MP3, you might actually lose more of the file.
On the flip side, lossless files come in their original and uncompressed format. Just like in the case of zip files, lossless compressions tend to shrink the size of the audio package without getting rid of anything. Unlike in the case of MP3 files, FLAC comes in lossless format, which can be converted back to its WAV format, where it can be restored to the original format.