If you don't know what the term width refers to, please read first this answer.

Particles can be caracterized, in particular, by their mass m and their decay lifetime Δt. The decay lifetime is the average time interval taken by the particle to decay to lighter particles. The lifetime of a particle can be infinite (e.g., for the electron) if there is no lighter particles to decay to, but can also be very small (e.g., for the Higgs boson or the Z boson) in case many decay channels are open. If the lifetime Δt of a particle is small, the Heisenberg principle tells us that its mass m is uncertain:

Δt x Δm ~ h/2π