I believe threading is a unixODBC specific parameter for driver threading awareness.
For reader convenience, this is the source comment... apparently higher values will mutex your driver into oblivion ;-)
242 /*
243 * use just one mutex for all the lists, this avoids any issues
244 * with deadlocks, the performance issue should be minimal, if it
245 * turns out to be a problem, we can readdress this
246 *
247 * We also have a mutex to protect the connection pooling code
248 *
249 * If compiled with thread support the DM allows four different
250 * thread strategies:
251 *
252 * Level 0 - Only the DM internal structures are protected.
253 * The driver is assumed to take care of itself
254 *
255 * Level 1 - The driver is protected down to the statement level.
256 * Each statement will be protected, and the same for the connect
257 * level for connect functions. Note that descriptors are considered
258 * equal to statements when it comes to thread protection.
259 *
260 * Level 2 - The driver is protected at the connection level. Only
261 * one thread can be in a particular driver at one time.
262 *
263 * Level 3 - The driver is protected at the env level, only one thing
264 * at a time.
265 *
266 * By default the driver opens connections with lock level 0; drivers
267 * are expected to be thread safe now. This can be changed by adding
268 * the line
269 *
270 * Threading = N
271 *
272 * to the driver entry in odbcinst.ini, where N is the locking level
273 * (0-3)
274 *
275 */
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