Your muscles work by contracting little strings, which transfers energy. They've got a limit for how much energy per distance-contracted (aka force) they can apply.
Levers and pulleys are simple devices that move one thing a certain distance while another thing moves a different distance, but transfer the energy while doing so. They let you transfer the same amount of energy over a larger distance, so there's less energy-per-distance (force), so your muscles can cope with it. (They have to do more contraction-expansion cycles, but it's easier because there's less energy going into each of them.)
Levers and pulleys are simple devices that move one thing a certain distance while another thing moves a different distance, but transfer the energy while doing so. They let you transfer the same amount of energy over a larger distance, so there's less energy-per-distance (force), so your muscles can cope with it. (They have to do more contraction-expansion cycles, but it's easier because there's less energy going into each of them.)