Enzyme kinetics; what happens at the peak of the Gibbs energy graph?
Enzymes are catalysts that lower a reactions activation energy (Ea), and thus increases the rate of the reaction. The diagram above is a Gibbs-Free energy landscape/pathway, where the top of the diagram is the transition state between the substrate and the product. Enzymes speed up chemical reactions by stabilising the transition state, and in effect, lowers the activation energy (Ea) required to transform the substrate into the product. The active site of enzymes are thus specifically 'designed' to be as energetically favourable as possible for the transition state.

To understand enzyme kinetics it could also be a good idea to look at (understand on a general level) the Arrhenius equation and reaction rate constants (k), to more completely understand what we mean by activation energy and reactions rate.
This whole topic is taught in basic biochemistry, and thus well described by many educational websites, all biochemistry text books, and even found in  educational youtube lectures (which is perfect for a basic understanding before you read more heavy theory). My earlier biology SE question Enzyme kinetics: recommended literature to grasp the concepts better also has some good book recommendations.
Following are also some webpages you can look at.
Khan Academy:

With the catalyst, the activation energy is lower than without. ...
That's because enzymes don't affect the free energy of the reactants
or products. Instead, enzymes lower the energy of the transition
state, an unstable state that products must pass through in order to
become reactant (...)

Chemistry LibreTexts: General transition state stabilisation is also a nice keyword that would give you a lot of information, e.g. from Chemistry LibreTexts:

Linus Pauling postulated long ago that the only thing that a catalyst
must do is bind the transition state more tightly than the substrate.
That this must be the case can be seen from the diagram (...)

Wikipedia: It could also be a good idea to generally read about catalysts, here from the wikipedia page where they use the classical example chymotrypsin and its oxyanion hole (active site that binds the substrate) that contains the Asp-His-Ser catalytic triad:


In the presence of chymotrypsin, (...), a nucleophile is used in the
form of the catalytic triad - Asp 102, His 57, Ser 195 side chains.
Moreover, the oxyanion hole, which consists of the backbone -NH-
groups of Gly 193 and Ser 195 of the enzyme, have the N-H groups
positioned in such a way that they will donate strong hydrogen bonds
to the substrate's C=O oxygen, given that the carbon atom is
tetrahedral as found in the transition state. This strains the bonds
of the trigonal planar C=O of the original substrate, helping the
reaction to proceed to the transition state. The hydrogen bonds also
stabilize the formal negative charge on the oxygen atoms. In this way,
the activation energy of the reaction is lowered and the rate of
reaction thus increases.