Stoichiometry is a very basic chemistry concept. It is just a way of measuring or determining the amount of each substance that is involved in a reaction (reactants), and the amount of products that are generated.
Before actually running a reaction in the lab, a chemist needs to figure out what is the number of molecules of each reagent is required for the reaction to proceed. For this purpose, we use a unit called “mole”. The mole is the base unit of “amount of substance”. One mole accounts for 6.022·1023 molecules. We need it to be a huge number, since there is a huge number of molecules in each gram of any reagent of a reaction.
The stoichiometry of a reaction is the measurement of the relative quantities (or equivalents), measured in moles, of the reactants that are involved in the reaction.
For instance, each 2 molecules (or 2 moles) of hydrogen gas (H2) react with 1 molecule (or 1 mole) of oxygen gas (O2), to generate 2 molecules (or 2 moles) of water (H2O):
2 H2 (gas) + 1 O2 (gas) → 2 H2O (liquid)
So if we were to perform this (rather inpractical, using two expensive, difficult to handle gases, to get a cheap easy to find product such as water) reaction in a lab, we would have to mix together 2 moles of hydrogen per mole of oxygen. This means using 2 equivalents (equiv) of hydrogen respect to the amount of oxygen. In this case, while handling gases, the number of moles of each can be controlled by establishing a partial pressure for each of them.
In case of more common solid reagents, we can consider a hypotetical synthesis of sodium carbionate from carbonic acid. We need 2 equivalents of sodium hydride (NaH) per mole of carbonic acid employed (twice as many molecules of NaH than molecules of carbonic acid).
This would yield 1 equivalent, or 1 mole of sodium carbonate, and also release 2 equivalents (or 2 moles) of hydrogen gas as a byproduct.