What is an unstable nucleus emits

Unstable nucleus tends to emit small particles of matter (alpha or beta particles) with an attendant emission of electromagnetic radiation and some amount of energy.

For every atomic nucleus, there is a specific neutron/proton ratio which ensures stability of the nucleus.

Any nucleus with a neutron/proton combination different from its stability ratio (i. e either too many neutrons or too many protons) will be unstable and split.

The will emit some small particles and radiations until they become stable.