Deadwooding is a process of removing dead or rotten branches. Rot can fester and spread to other branches and even the main tree trunk.
Your tree would be in danger of dying if these branches are not removed. Heavy branches that have died or are infected with rot are in danger of breaking off at any time, potentially causing damage to your property or person should you be walking under the tree at that moment.
For safe removal of dead or rotting branches specialised equipment will be used and should ideally be carried out by a certified tree surgeon. If the whole tree has died, you may require tree removal services.