This is the 14794th most frequent Russian word.
плеча
"Shoulder" (singular, genitive case in Russian).
Here, 'плеча' (of/shoulder) is in the genitive case, indicating possession ('с плеча', from the shoulder).
Поправь сумку, она сползла с плеча.
Adjust your bag, it slid off your shoulder.
The word 'плечо' (shoulder) appears in the accusative case ('на плечо') as part of the verb phrase describing movement.
Мы взгромоздили тяжёлый ящик на его плечо.
We placed the heavy box onto his shoulder.
The word 'плечо' (shoulder) is in the accusative case ('плечу') as it is part of the complement of the verb 'похлопать' (to pat).