To let thoughts run freely — meditating, musing, or speaking at length. Used of both deep meditation on God's word and of complaint poured out in prayer. Both are valid forms of speech to God.
The range from meditation to complaint reflects honest prayer. Psalm 77 uses siach for troubled musing in the night; Psalm 119 uses it for delighting in God's statutes. Same word, different heart postures.
The word's breadth — meditation and complaint — validates the full Psalter's range. God welcomes both the praising meditator and the lamenting complainer. Both are forms of authentic engagement with the living God.