a wire mesh or perforated metal utensil used for straining or sifting.
is very commonly used metaphorically to describe things that are leaking or something that has holes in it.
1) leaks like a sieve
2) bad defense in sports (i.e. poor goalie in hockey)
1) The pipes are leaking like a sieve. The plumber must have done a crappy job.
2) That goalie sucks. He is about as porous as a sieve