Self Doubt ( Decrease key operation )

Question

While going through dijkstra's algorithm, there is a term "decrease key". I am not getting the meaning when it says "we do decrease key operation". What exactly we do and what is the meaning of decrease key ?

Comments

We decrease key during relaxation.

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@Rishab Gupta 2 thats what i am asking, what exactly we do during decrease key ?

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we need to decrease the distance we calculated to a vertex if we have found a path to reach that vertex in a shorter distance.
What I mean to say is, suppose currently the distance to reach a vertex v is d[v], from source node s,
Later on, we found that we can reach v in a shorter distance. To check this we do relaxation, i.e.
for a vertex, u, with distance from source vertex d[u], and v is connected to u with an edge of weight x, then
if(d[v] > d[u] + x)
This checks the condition if the previous distance to v is more than the distance of s to u and then u to v.
If d[u] + x is less than d[v], then it says that we can reach v, from s, in a shorter distance than previously calculated in d[v]. so we change d[v] to d[u] + x, i.e. d[v] = d[u] + x, and change the parent pointers accordingly.
This is relaxation.
We generally use a min heap. And whenever we do the above step, we are decreasing the value of d[v]. This is decrease key.