The first step in acyl substitution is an addition step, forming a tetrahedral intermediate. This is exactly like the first step in the addition of nucleophiles to aldehydes and ketones. In the second step, the tetrahedral intermediate loses the LG and the carbonyl is reformed. The net effect is a substitution.
In this case, the Nu is negative ( -OH, -OR, -R, or H-) so the nucleophilic atom will become neutral.
The exact mechanism followed and the nature of the tetrahedral intermediates depends upon a number of factors including 1) nature of the carboxylic acid derivative, 2) the nature of the nucleophile and 3) whether under acid/basic nucleophilic conditions.
If the nucleophile is protonated and neutral (HOH, HOR, NHR2) then it will become positively charged in the tetrahedral intermediate. In such cases, a deprotonation occurs.
For poor nucleophiles such as H2O or HOR and less reactive esters and amides, acidic or basic catalysis is often required.
For example, water will not readily undergo addition to esters, under neutral conditions. The addition is energetically unfavorable.
Protonation of the carbonyl prior to nucleophilic attack "activates" the carbonyl for nucleophilic addition. A protonated carbonyl is a better electrophile. Can you draw the other resonance structure of the following protonated ester?
Under basic or nucleophilic conditions such as with NaOH or NaOR, the -OH or -OR, is a sufficiently strong nucleophile to attack directly.