IPA: /ˈʃæl.ə/
KK: /ˈʃæl.ə/
Having less depth than something else; not deep.
The water in this part of the lake is shallower than in the middle.
Shallower is formed from "shallow" (from Old English "sceald", meaning not deep) and the suffix "-er" (meaning more). The word "shallower" means more shallow or less deep than something else.
Think of something that is 'not deep' ('shallow') and remember that 'shallower' means more of that quality.