What are the effect of electrical cable joint?

A cable joint is the term used in the UK for what, in North America, is called a cable splice.

In the case of high- and medium-voltage underground cables, if they are done correctly, there is no adverse affect and joints should last as long as the cable itself. A great deal of care is taken replacing the insulation after jointing the conductors themselves, and to ensure that the cable remains waterproof and any armouring is replaced. HV and MV cable jointers are highly-skilled tradesmen who have completed an apprenticeship in cable jointing.

The connection of wires is typically called a splice (North American term). If the splice involves two different types of wire like copper and aluminum you would have to use special connectors made for this purpose and perhaps a compound made for the purpose. In the normal connection of two or more wires of the same type you need to make sure the ends of the wires are clean and that the connection is firm so as to reduce resistance at the joint. You also need to make the splice in a junction box for power wiring applications; and that bare wire isn't exposed which could cause shock or shorting situations. Underground splices pose yet another problem with moisture and varmints chewing on the splice.