The semi-short answer is an input is an incoming amount to an address.
Well I'm thoroughly flummoxed now...
My wallet now holds 1001.9 TZC obtained through 192 mining payments to 2 addresses, and the last payment was a little over 48 hours ago so the minimum age of an input should be >2 days. The Penrose triangle, however, reports 148 inputs weighing 10178 coin days, of which 52 inputs are minimum age, 1 input is average age and 95 inputs are max age; so 148 inputs vs. 192 payments, and last payment was 2 days ago but still claiming that 52 inputs are minimum age. To muddy matters even more, the TZC whitepaper says that the minimum age for an input is 1 day, so is it any wonder that I am confused at this point?
Hopefully you checked up on inputs/outputs/etc., as that will help any communication/understanding.
Every time your wallet has received a payment, for example a small mining pool payout, that will represent an input. That is probably why you have so many inputs.
Note that inputs, while associated with addresses, are not 1-to-1 with an address. You can have many inputs under the same, single address.
The min, ave, max age are just a way of saying low, medium, high maturity. I don't remember the exact breakdown, but maybe inputs from 1-8 days in age may be min, 8-16 days ave, and 16+ days max? Anyway, it's just a 3-bucket way of considering your inputs and their age.
Your inputs that are under 24 hrs old (or perhaps 72 hrs if you don't have a conf on the original version wallet) are probably not being included in the count (e.g., the 148 you mentioned). Also, there may be a minimum input size to stake, but I have not gotten solid confirmation on that. So if that is true, inputs that are too small would not count either, theoretically.
Based on what you seem to be describing--a fair likelihood of many small inputs--it might be worth taking a moment to sweep up the tiny ones into larger inputs. You will reset the ages of any inputs you merge, but I think it would probably be worth it. Please be aware that regardless of whether they are set right or not, a total coin balance of 1000 will probably take months or longer to hit a PoS stake--of course, if you are particularly lucky it could happen much sooner.
Hope that helps--best of luck with your staking!