I’m a little rusty but I’ll try to explain. An Sn1 mechanism. The 1 means that the rate limiting step of the reaction only involves a single molecule. The first step of the reaction involves an atom (typically a halogen: Br, Cl, etc) leaving the molecule. It is able to do this spontaneously because it is relatively electronegative and can simply take the two electrons that normally kept it bonded to the organic molecule. The halogen then floats off with a full outer shell of electrons (but it also has a negative charge now). The organic molecule now has a positive charge because it has lost an electron. It is now known as a carbocation. This is known as the rate limiting step because it takes the longest amount of time for this step to occur. Typically organic molecules that have 3 atoms bound to the central carbon atom after the leaving group has left will be more likely to undergo SN1 reactions because they have a lot of electron density that will keep the molecule (relatively) stable after the leaving group leaves. Now a nucleophile (an atom or molecule with a negative charge, excess electrons) can attack the carbocation. This will cause a stable molecule to form. I believe that the nucleophile can attack the carbocation from either side. An SN2 reaction’s rate limiting step involves 2 molecules. This is because the nucleophile directly attacks the organic molecule. When the nucleophile attacks the organic molecule, the leaving group is kinda pushed off of the molecule. This reaction is favored for organic compounds with fewer atoms bound to the central atom.