This is when we use an indirect object pronoun and a direct object pronoun together in a sentence. If we use an indirect object pronoun and a direct object pronoun together; the placement of the indirect object pronoun is first and direct object is second.
E.g. me lo das = you give it (masculine singular item) to me.

For both 3rd person singular and plural the le becomes se. E.g. se lo diré – I will tell it to him/her.
Ideally you should include a specific reference to the person/people to whom you are referring in the 3rd person forms of the verb for clarification. E.g. se lo diré a él/ella.

Remember to use a direct object pronoun, you must have mentioned the direct object first!