its defined by pairing its ten syllables per line into five pairs, and in kind of a stressed and unstressed pattern. Sounding like 1[da Dum] 2[da Dum] 3[da Dum] 4[da Dum] 5[da Dum] (the numbers represent pairs, the brackets are for organization) i did a terrible explanation, but thats kinda the gist, its mainly used in poetry, and pentameter also refers to meters which are patterns of syllables in a line, and since in pentameter, theres five