Economic variables can be "real" or "nominal". So when policymaker discus influencing the economy, they look at implications of influencing these economic variables.
An example of a nominal variable is the price level influenced by money supply. Money is directly created by the government and can take on any value per se and is not "real. E.g. if we multiply all prices and bank notes by 10 nothing in the economy changes.
Real variables are for example output, productivity etc.
However, it turns out that these nominal variables can influence the real ones. Increasing the money supply can increase output (at least temporarily). This is also easy to implement, so monetary policy is often used to fight a recession. Note that lowering the interest rate and increasing the money supply are two sides of the same coin and both part of monetary policy.
An alternative to monetary policy (based on nominal variables) would be "real" economic reforms. One form of "real" policy is a "structural reform". In this case you would try to increase output by changing something in the real side of the economy. For example lowering (wage) costs through changes to labor protection laws. Fiscal policy (government spending or tax cuts) is another form of "real" policy, however it is not a structural break.
To fully understand structural reforms we must introduce the concept of output potential. This is the output that would normally occur in absence of disturbances. It only depends on the underlying structure of the economy. These are factors such as costs (e.g. ease of doing business), labor market flexibility and other things that influence output in general in "real" terms. The economy generally comes back to that level and fluctuates around it in the medium run. (Note this level may be growing over time, e.g. by 1-2% each year).
However, sometimes it happens that our economy is operating below its "natural" potential. In this case we can employ monetary and fiscal policy to get back to that natural level. We can also push the economy beyond the natural level, but we will eventually come back, so this is not a good idea since the side effect that brings is typically more inflation.
If the crisis is because we are simply below potential, we do not need structural reforms. However if we are in a crisis because the potential or "natural" output has risen, then we need "structural reforms". An often cited recent example is the Greek crisis where Greece's wage rate increased a lot over time making its exports too expensive, meaning the economy did not have its past potential to export anymore. In this case many economists suggested "structural" reforms (law changes) that would lead to lower wages.
Monetary and Fiscal policy are often used to correct short-term problems or deviations from output potential. Structural reforms are supposed to change the (medium to long term) output potential itself.
The type of policy best suited often depends on the type of crisis at hand and its underlying causes.