Pesa vs. peso
The Spanish words 'pesa' and 'peso' both relate to weight but are used differently.
Pesa
The word 'pesa' typically refers to a physical object used for exercising, like a dumbbell or weight.
Voy al gimnasio y levanto pesas todos los días.
(I go to the gym and lift weights every day.)
Compré unas pesas nuevas para hacer ejercicio en casa.
(I bought some new weights to exercise at home.)
Las pesas están demasiado pesadas para mí.
(The weights are too heavy for me.)
Peso
The word 'peso' refers to the concept of weight as a measurement of how heavy something is. It can also mean the currency used in several Spanish-speaking countries.
Mi peso ha aumentado tres kilos en el último mes.
(My weight has increased by three kilos in the last month.)
El peso del paquete es de cinco libras.
(The weight of the package is five pounds.)
El peso argentino ha fluctuado mucho este año.
(The Argentine peso has fluctuated a lot this year.)
Summary
'Pesa' usually refers to an object like a dumbbell, while 'peso' relates to the measurement of weight or currency in some Spanish-speaking countries.