Think about the story of Goldilocks and the Three Bears. That is what ecological tolerance is all about; finding what, per ecosystem, are the ideal conditions for survival and prosperity. Most organisms exist in their environment simply because it benefits them the most to stay in those conditions. A species that is able to tolerate a wide range of environmental conditions is said to have a high ecological tolerance, while a species with a narrow range of tolerated conditions is said to have a low ecological tolerance.
All populations need certain factors to survive. In no ecosystem is there zero food, zero water, or zero shelter. Although, what species would require an environmental change if their access to sunlight was stifled? Or, if soil composition was different? What about an ecosystem's proximity to predators or competition?
If there is slightly too much or too little of an environmental factor, the population size will decrease because individuals are stressed. Time and energy is delegated away from reproduction or care for offspring and instead directed at survival and self-care. Reproductive rates will plummet and the ecosystem is faced with intolerance, when members die off or leave due to conditions being unfavorable.