Infiltration describes entry into a space or material. In building science it often refers to outdoor air entering through gaps; in water discussions it may refer to water entry.
What this means at your property
A window complaint might involve air leakage, rain passing a joint or condensation. Those mechanisms call for different repairs. An estimate should say which kind of infiltration was observed and where the entry path is believed to be.
What should be checked before sealing a gap?
Ask how the source was identified and how the proposed repair affects adjoining drainage or ventilation paths. Covering a visible opening without understanding the assembly can leave the original problem unresolved.