An economic stall is a situation where the total expenses for a resource has overcome production and has completely depleted storage amounts. What is common about this is that it slows production of anything being built at the time to compensate for the fact that there is only a limited amount of income for said resource available. Running out of different resources, however, will produce different effects.
When running out of mass, or Mass Stalling, no intel or defensive structures are disabled. However, the effects are longer lasting as one must pause production of some or all of the units being built (engineers, factories and upgrades alike), thus leading to a significant or complete slowdown in expansion.
When running out of energy, or Energy Stalling, the effect is more instantaneous as what causes your energy to go negative often does so by a large amount, thus quickly sapping your reserves. If the drain is severe enough, it will cause radar systems to shut off, artillery structures to take longer to fire, and even shut down your shields. However, since several kinds of structures an units consume energy, it is easier to correct the situation by shutting some of them down or pausing production and upgrades.
New commanders may struggle to fix an economic stall, by building mass extractors or energy generators, using up resources thus further tightening the situation. Always be prepared to pause or shut off any unit in case of a stall. Having a proper build order helps diverting early game stalls and late game stalls are avoided by making proper assessments of your economy before starting additional constructions, such as when upgrading your mass extractors. In some cases an economic stall is inevitable, usually while building experimentals or game enders.