Often the terms "being machmir" in contrast to "being lenient" are thrown around. What does it mean? The assumption is, that we have a question where halacha is not clear cut and there are several valid opinions. We can then either decide according to the strict interpretation (machmir) or to the lenient interpretation (makil). An example would be the waiting time between meat and milk. Normative halacha in my place is 3 hours [this is German minhag as held in my community, the number doesn’t matter, it is just an example]. If I were to rely on authorities that say it is enough to wait 1 hour (Dutch minhag), I am being makil (lenient). If I rely on opinions that require 6 a hours wait (e.g., Chabad), I am being machmir (strict). If I take onto myself to always be machmir in this point, I am taking on a chumra.
The important point here is, that being machmir means going beyond normative halacha. It is perfectly fine not to be machmir. So in the above example, if somebody is holding 3 hours, this is perfectly ok. Holding 3 hours in this case is also not lenient, it is just halacha.
There is nothing inherently wrong with being extra careful in matters of halacha. But it is only "being machmir" if one is following a valid opinion that says to be more strict on this point. I am not being machmir if I refrain from buying green sponges and buy only red or yellow ones. There is no basis for that in any halachic source, so I am just being restrictive for no reason. The feeling in haredi society seems to be that more restrictive is always better, even if there is no real reason to restrict anything. Examples for this type of chumra can be found in many blogs, mostly in the area of tzniut (stockings, photos, separated seating in busses, …) or kashrut (two ovens, banning strawberries, tap water, …).
Many people seem to think that being orthodox is to be restrictive about everything, always. Don’t get me wrong, I think everybody is free to take on any chumra he/she likes. Many of them are there for actual halachic reasons. The problem is, when the chumra is presented as normative halacha and what other people consider normative halacha is suddenly a "leniency". This is just not correct and drives me mad.