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I'm assuming you're talking about SN2 reactions.

Alcohols are of the formula R-OH, where R is any carbon group.

Alkyl halides are of the formula R-X, where R is any carbon group, X is any halogen (except for F, they're much less reactive)

When you react R-OH and R-X, what you want to occur is the electrons on the oxygen attack the carbon on the R in R-X to displace the X. However, the oxygen in R-OH is a poor nucleophile. To improve its nucleophilicity, you can deprotonate it, giving it a negative charge in order to improve the speed/success of the reaction.

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