It depends on the context, to some extent, but the concept is pretty self explanatory:

• recurrent - seen more than once in a cohort of patients (often to some degree of signficance)
• de novo - new, not inherited
• mutation - change in DNA sequence, by point mutation, indel, SV, CNV, etc.

If you want more than this, you're going to have to give some better context

De novo mutation definition (http://ghr.nlm.nih.gov/glossary=denovomutation): An alteration in a gene that is present for the first time in one family member as a result of a mutation in a germ cell (egg or sperm) of one of the parents or in the fertilized egg itself.

A recurrent de novo mutation would be a de novo mutation that occurs repeatedly.

e.g. Haemophilia A is caused by mutation of the FVIII gene. Two different inversion mutations explain half of all severe cases of haemophilia A. The inversion mutations occurs repeatedly, i.e. de novo. They are caused by recombination between near identical copies of a DNA segment found within and outside of the gene, and in opposite orientation relative to one another (PMID:8275087; PMID:11756167).

Recurrent de novo point mutations can occur, for example, at CpG sites (PMID:25401298)