As soon as you start playing Warframe, you'll be overwhelmed by the number of various resources, items and currencies that you'll start collecting practically immediately, and no one could blame you if you missed the fact that a particular one keeps popping up as you go along, and this is - of course - Endo.
Endo is a mysterious substance left over by the Orokin Empire, and which is used to enhance mods - something that is absolutely necessary if you want to keep progressing normally through the game. For example, say you've got the base Serration mod, which increases your primary weapon's sheer damage output. Upon picking it up and slotting it into your weapon, it will provide a buff of 15% damage, but if you upgrade it three times, it'll provide a buff of 60% damage, which makes for a fairly substantial increase in DPS.
Similar as to how Credits work, you'll be getting Endo for doing anything and everything. Oftentimes, simply killing an enemy will drop a small stack of Endo for you to pick up, while other times you'll get large amounts for completing objectives or missions. Additionally, you can visit the Earth Relay station and speak to Maroo to get a weekly treasure-hunting mission, where you'll have to complete an environmental puzzle to get to a special Ayatan sculpture. Ayatan can be traded for substantial amounts of Endo, but can also be placed in your headquarters as a decoration, and there are about half a dozen different versions for you to pick up.
Back to the matter at hand, though - you will need large quantities of Endo if you want to keep your arsenal up-to-date and properly spruced-up. Take the Serration from the beginning of the article, for example. This is a silver ten-rank mod, and the cost of each successive upgrade increases exponentially. To fully max it out from rank 0, you will need 20 460 Endo and 988 218 Credits, which is a pretty large investment to make. It is for this reason that the suggested course of action is to keep upgrading your mods incrementally. For a practical example, getting Serration to rank 9 will cost only half of the aforementioned Endo and Credits required to get it to rank 10, with only 15% less of a damage increase.
With all of this in mind, Warframe requires you to weigh your options and put some thought into how and why you're upgrading your stuff. Endo is plentiful, but not plentiful enough to keep maxing-out every mod you find, and it's not like you're in any real rush playing a PVE game anyway.