Cats, especially ones who spend time outdoors, are susceptible to eye and ear infections. Your vet should be able to tell you. If it is an infection, the vet can prescribe antibiotics to be taken orally (pills) or applied topically (a cream to be rubbed into the eye).
If it is a clear discharge, it is likely caused by an allergy. If an allergen is getting in your cats' eyes, they will water to expel the agent. If there is a colored discharge, or if it is thick, it could be an infection.