In anr traces.txt what's 'held mutexes=' means?

In anr traces.txt file , what's 'held mutexes= ' means?

"main" prio=5 tid=1 Blocked
  | group="main" sCount=1 dsCount=0 obj=0x77d21af8 self=0x7fa2ea2a00
  | sysTid=20008 nice=-10 cgrp=default sched=0/0 handle=0x7fa6f4ba98
  | state=S schedstat=( 278831875 7233747 156 ) utm=22 stm=5 core=0 HZ=100
  | stack=0x7fd42e0000-0x7fd42e2000 stackSize=8MB
  | held mutexes=

"main" prio=5 tid=1 Runnable
  | group="main" sCount=0 dsCount=0 obj=0x77d21af8 self=0x7fa2ea2a00
  | sysTid=15564 nice=-10 cgrp=default sched=0/0 handle=0x7fa6f4ba98
  | state=R schedstat=( 22116939220 18299419 428 ) utm=2209 stm=2 core=5 HZ=100
  | stack=0x7fd42e0000-0x7fd42e2000 stackSize=8MB
  | held mutexes= "mutator lock"(shared held)