Hi Leona,
Welcome to CiC - it's great to have you with us
Not a lot to add to the above except to say that JPEGs are designed to be small so that they download and display faster on the internet.
In the world of photography, you'll want to keep 3 copies of each file ...
1. The original RAW file (or a DNG equivalent)
2. A full resolution version, saved will lossless compression (eg PSD for Photoshop, or TIFF from other programs), and
3. A down-sampled JPEG version, for uploading to online photo albums etc.
Does this help?