Salesforce out of the box means the declarative features provided by the Salesforce, where we can develop applications just by using button clicks and without any code e.g. workflow/approval process,reports and dashboards .

You can customize lot of things using OOTB features of Salesforce CRM. In terms of opportunities, following are possible with just click and NO Coding/ Programming
1. Look and feel of the layouts. Customize them with your preference.
2. Set your own business process / sales automation
3. Have email alerts for eg with big opportunities
4. Set security level for who should see what kind of opportunities. may be restrciting withing group or set role heirarchies.
5. Have some custom valdiations on Opportunities which would rquire adhering to companys requirement.
6. Pull out reports of the deals in progress, past deals and futue deals. Check out the forecasting feature and broader your management's vision on how to achieve more.

There are three things here:

- Out of the box functionality : 
Out of the box (or vanilla) functionality means the product capabilities that are available in a package/Software. E.g. Salesforce provides a lot of features that are available for the users to use directly without making any changes. This, of course varies by the edition of Salesforce.  E.g. there are features like Standard Reports, Standard dashboards, page layouts, standard objects and fields, administrative features etc. which can be directly used without any modification.

- Configuration: 
The Salesforce features that are available for use but as per your client requirements you need to add/modify e.g. when you want to create a new entity which does not exist in Salesforce you will create a new object and fields. When you want to create a validation rules which prevent something from happening you can use validation rules. You can use configuration to build Validation rules, Workflow rules, Approval process, formula fields, Custom reports and dashboards etc.

- Customization:
The features/functionality which are not available in Salesforce or  when you want to add functionality on top of the existing functionality you need customization. Salesforce provide you tools which are used to customize the application. These are Apex, Triggers, Visualforce, API (SOAP, Restful, streaming, chatter) etc. e.g. when you want to create a powerful validation rule which cannot be created via configuration you can use Apex Triggers (Customization). Similarly when you want to build a more powerful workflow rule you can use Apex Triggers (Customization) OR when you want to integrate your Salesforce application with external application you need customization (SOAP or Rest API) etc.

1) https://altvia.com/5-reasons-why-salesforce-out-of-the-box-doesnt-work-for-fund-management/

Let us know if this will help you