39 But the salvation of the righteous is from the Lord;
He is their strength in the time of trouble.
⁴⁰ And the Lord shall help them and deliver them;
He shall deliver them from the wicked,
And save them,
Because they trust in Him.

Salvation - deliverance, help, safety, salvation, victory. 

Righteous - just, lawful, righteous (man). 

Trouble - adversity, affliction, anguish, distress, tribulation, trouble.

Deliver - carry away safe, deliver, (cause to) escape. 

Trust - have hope, make refuge, (put) trust. 

But the salvation of the righteous is of the Lord - Or, salvation comes to the righteous from the Lord. While the wicked are cut off, the righteous shall be safe. There are evidently two ideas here:

(1) that there will be salvation to the righteous, while the wicked are cut off;

(2) that this comes from the Lord, and not from themselves.

It is not owing to any power of their own that they are safe,but is solely because they are kept by the Lord.

He is their strength in the time of trouble -

And the Lord shall help them - He will interpose to defend them when they are in danger and in trouble.

And deliver them - Rescue them from their dangers, and from the power of the wicked.

He shall deliver them from the wicked - From all the attempts of the wicked to destroy them.

And save them - Or,preserve them. He will keep them to everlasting life.

Because they trust in him - They rely on him, and not on themselves. We should not fret,or complain at the prosperity of wicked men. The thought in this psalm is, that, whatever prosperity the wicked now have, it is temporary, for they will soon be cut off; and that whatever troubles now come upon the righteous, they too are temporary, and that their "hereafter" will be blessedness and peace. 

God is the enemy of the wicked. It is better to have God for our friend in life, and our support in death, than to have all the external prosperity of wicked men.

Trust the Lord for He is a just God and by trusting Him and Him alone we will spend eternity with Him in Heaven.

(Some insights taken from Barnes' Notes)