let me help you explain it a bit more...
To level up your warframe, so you can add mods, you need to grind levels on it.
Usually the best maps for this is the higher level defense maps, or the orklin defense maps, where basically ur gaurding something in the middle from a mindless wave of mobs which spawn.
This is where warframe class and mods are very important... example... some maps u want a Nova with uber slow... other maps u want a crack nova, where it speeds up the mobs.
Then u want a frost to pull up a defense shield, while a necro to loot the dead mobs for mods.
Mods are in the form of cards which need to be upgraded by combining.
Some of the mods will require at least a 1000 cards to combine, or something called a fusion card. In all, its a grind to max out mods.
Warframes can only take certain mods unless u use a item to change the polarity of it so it reduces the cost of mods by half. Once u change a polarity of anything, it resets the level of the warframe / weapon back to 1, and again, you need to grind the hell out of it to max its level.
An example is like my Nova.... ive litterally had to grind 5 level resets out of it, to get the perfect polarities.
Now lets say u want a new warframe.
The easiest way to do it is to buy it via premium currency.
The hard way to do it is to buy a extra warframe slot + wait for alerts so you can get the orkin cell schematic to super charge it, craft the orkin cell.
Then you need to grind the map which the warframe schmatic drops from collect all of the schematics, then collect all the resources required by again, grind the map which associates with the resources + finally time gate wall, in which its being created. Messy huh?
Hence why i said its best to buy it with premium currency because ur given the free warframe spot + the orkin cell + the warframe itself.
Its about at least a 4 day savings in time, assuming u dont have the schematics and need to camp the alerts.
As i said, the game is a massive grind.... hence why people i know all get burnt out really quickly. Ive litterally grinded maps about 500 times to get all my mods maxed out and that is not fun.
Also lets not even get into fraction, and fraction mods.... sigh.... my head exploded when i realized how much grinding that required.
Also u do not get experience unless ur leveling something... so when ur sitting on all maxed items, you need to swap to something with no levels, or your avatar experience will not increase.
Also level promotion tests get significantly more difficult and i find it unfair that some warframes can actually cheat some of them... Ie.. Nova's teleport can make some of those tests rediculously cheap, by teleporting to the finish line. :thumbsdown: