Reversible reactions

Your first clue is the arrow. When you see →, you should read it as a reaction that, under normal conditions, goes to products and more or less stays there. When you see ⇌, you should expect a tug-of-war where your products can turn back into reactants as well.

An

These reactions often give out a lot of energy and leave you with products that are especially stable. That is why burning reactions are usually treated as one-direction processes.

For example, methane burning is written exactly as:

${\mathrm{CH}}_4(\mathrm{g})+2{\mathrm{O}}_2(\mathrm{g})\to {\mathrm{CO}}_2(\mathrm{g})+2{\mathrm{H}}_2\mathrm{O}$

Once you have carbon dioxide and water, they do not normally join back together to remake methane and oxygen. Another familiar one-way example is:

$2\mathrm{Mg}(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})\to 2\mathrm{MgO}(\mathrm{s})$

When your magnesium burns, the magnesium oxide formed is very stable, so you do not see it casually splitting back into magnesium and oxygen.

A

In the same container, both directions can happen at once. That sounds weird at first, but it is the whole point of equilibrium.

A clear example is:

${\mathrm{H}}_2(\mathrm{g})+{\mathrm{I}}_2(\mathrm{g})\rightleftharpoons 2\mathrm{HI}(\mathrm{g})$

If you seal this in a closed container, hydrogen and iodine make hydrogen iodide, while hydrogen iodide also breaks apart back into hydrogen and iodine.

Constant does not mean equal amounts , just no overall change.

One surprising bit: at equilibrium, your reaction has not stopped. It only looks unchanged from the outside. On the particle level, collisions and reactions are still happening both ways.