sich ansammeln
to accumulate
Something increases on its own over time or piles up in one place.
In der Dachrinne hat sich viel Laub angesammelt.
A lot of leaves have accumulated in the gutter.

to accumulate / collect
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to accumulate
Something increases on its own over time or piles up in one place.
In der Dachrinne hat sich viel Laub angesammelt.
A lot of leaves have accumulated in the gutter.

'Ansammeln' (to accumulate / collect) is a separable verb with the prefix 'an-'. In a main clause, 'an' stands at the end of the sentence.
Über die Jahre sammelte er ein großes Vermögen an.
Over the years, he accumulated a large fortune.
The use of 'ansammeln' shows whether something happens actively or passively. 'Sich ansammeln' (to accumulate, reflexive) means that something piles up on its own. 'Etwas ansammeln' (to collect something, with an accusative object) means that a person actively gathers something.
In der Ecke hat sich Staub angesammelt.
Dust has accumulated in the corner.
When something accumulates on its own, it is often something unwanted. Typical examples are: Staub sammelt sich an (dust accumulates), Schmutz sammelt sich an (dirt accumulates), or Schulden sammeln sich an (debts accumulate).
Nach dem Regen hat sich Wasser im Keller angesammelt.
After the rain, water accumulated in the basement.
to collect / gather
'sammeln' and 'ansammeln' both have to do with things increasing in quantity. 'sammeln' is usually used when someone actively and deliberately gathers something, for example coins, data, or experiences. 'ansammeln' puts more emphasis on more and more of something developing over a longer period of time or lying in one place. Very often people say 'sich ansammeln' when it happens more or less by itself, for example dust, work, or water. Learners may confuse the words because both describe a growing amount. If a person consciously takes and keeps something, 'sammeln' fits better; if something builds up over time, 'ansammeln' fits better.

Nach dem langen Regen hat sich Wasser im Keller angesammelt.
After the long rain, water accumulated in the basement.
Meine Nachbarin sammelt alte Fotos aus der ganzen Straße.
My neighbor collects old photos from the whole street.