Internal conflict is within a character’s mind and external conflict is outside of it.
Conflict is where there is/are opposing force(s) acting against the interest or wishes of a character. The difference between internal and external conflict is where those forces are and where they are acting.
Internal conflict is where the forces are within the character’s mind. For example, if a character has a substance abuse problem, the internal conflict could be between the character and the addiction (or perhaps between the character and the desire to be clean and sober).
External conflict is where the forces are outside of the character’s mind. For example, if we take the same character as above with the substance abuse problem, there could be conflict with the dealer (who is actively seeking to keep the character hooked on drugs) and loving family members (who are actively seeking to get the character clean and sober).