A resting spore is a spore created by fungi which is thickly encysted (has a thick cell wall) in order to survive through stressful times, such as drought. It protects the spore from biotic (microbial, fungal viral), as well as abiotic (wind, heat, xeric conditions) factors.
Resting spores create the phenomenon known as late potato blight. They can lie dormant within the soil of a field for decades until the right conditions occur for viability (plant host present, rain, fire etc.).