allylic cation
An allylic cation is a carbocation — a carbon bearing a positive charge and only three bonds — that happens to sit right next to a carbon-carbon double bond. That neighbouring double bond is not a bystander: it shares its pi electrons with the electron-hungry positive carbon, and in doing so it spreads the positive charge across more than one atom, which is a very stabilizing thing to do.
Here is the picture in slow motion. The positive carbon is empty in its p-orbital; the adjacent double bond's pi electrons slide over to fill part of that gap, which simultaneously moves the double bond and the positive charge to the other end of the three-carbon unit. We capture this with two resonance structures, CH2=CH-CH2+ and +CH2-CH=CH2, and the true species is the hybrid of both: a delocalized cation whose positive charge is shared equally over the two terminal carbons, with a single pi cloud over all three. Because no single atom carries the full charge, the system is much more comfortable.
This stabilization is why allylic cations behave like a higher class of carbocation: a primary allylic cation is roughly as stable as a secondary alkyl cation, and so on up the ladder. That has real consequences — allylic substrates ionize and react faster in SN1-type reactions, and because the charge lives at two carbons, a nucleophile can attack at either end, sometimes giving a mix of products (this is the same delocalization behind 1,2 versus 1,4 addition to conjugated dienes). It is a textbook case of resonance turning into reactivity.
When 3-chloro-1-propene (CH2=CH-CH2Cl) ionizes, it forms the allyl cation, whose positive charge sits equally on both end carbons; a nucleophile can then attack at either one, which is why allylic chlorides are unusually reactive in SN1 reactions.
Resonance shares the positive charge over two carbons, so the allyl cation is far more stable than a plain primary one.
The two resonance structures are not two molecules interconverting; the real allyl cation is one delocalized species in which both end carbons carry an equal partial positive charge at all times — the curved arrows show electron pairs moving on paper, not atoms hopping.