A bolt is a cheap method of anomaly detection available in all S.T.A.L.K.E.R. games.
This item is literally just a rusty metal bolt. It is unable to deal any damage and NPC's will generally not react to it (the exclusion is for the neutral guitarists in Shadow of Chernobyl, who will re-equip the guitar while the guitar music will still be playing). The purpose with bolts is the detection of anomalies. Since most, if not all, anomalies react to objects entering their area of effect, throwing a bolt into an active anomaly will trigger it. This helps in navigating anomaly fields or detecting the precise location of anomalies in conditions of poor visibility. Some anomalies, such as Springboard and Electro, can actually be passed through for a moment after being triggered. Thus, bolts can be used to clear a path through certain anomalous areas.
The player has bolts in the inventory at the start of the games and can never run out of them.