صحاري means " deserts "

صحراء means " a desert "

Note:

صحاري = [a deserts]=[sahaaree]

الصحاري = [the deserts] = [ a-ssahaaree]

صحراء = [ a desert]= [ sahraa2]

الصحراء= [the desert] = [a-ssahraa2]

صحاري = [sahaaree] |ee like in cheep, h special sound between h as in holiday and a sound|, h = ح ; it's plural of the word "desert"  [deserts].

صحراء = [sahraa2] |h as I explained before, and 2 i make it to mean the "a" sound but it's not large, we keep our voice on it, and then you hear "a" in the final; it means " desert.I am sure your book wanna talk to you about making plural in Arabic, it's important, good look with it.