An index is a calculation; a fund has to actually buy things, pay costs and deal with tax. The tracking difference is the gap between the fund's return and the index return over a period — usually a little below, mostly because of charges. Tracking error is how much that gap jumps around from one period to the next: a fund can have a small gap on average and still wander widely.
Check which index return the fund is being measured against. A price index ignores dividends, while a total return index reinvests them, before or after tax withheld abroad, so the same fund can look better or worse depending on the comparison. Neither figure appears in the app, which tracks your fund's price and distributions rather than its index. The fund's factsheet reports both.
Mostly costs. Tracking error measures how much this gap swings from period to period.
Educational content only. It is not personal financial advice.