The keren is the money the bank transfers to the seller. On an indexed track (Katz, Matz, Zakaut) the outstanding keren is adjusted every month by the Madad, so the amount you owe can rise even while you pay. How fast the keren falls depends on the repayment schedule: Spitzer (equal payments) repays it slowly at first, keren shava (equal principal) repays the same amount every month.
For an oleh or a first-time buyer the keren is tied to your equity: keren = price minus own funds, and Directive 329 caps it at 75% of the price for a single home. Every shekel of equity you add lowers both the keren and the interest you pay. You can also repay part of the keren early, which may carry an early repayment fee.
The pitfall is looking only at the first payment. On a 25-year Spitzer loan the first years barely reduce the keren, so if you refinance or sell early you still owe almost the whole amount.
Example
On ₪1,200,000 at 4% over 25 years (Spitzer) the monthly payment is about ₪6,334. In month 1, ₪4,000 of it is interest and only about ₪2,334 reduces the keren, which falls to about ₪1,197,666.